This application claims the benefit to priority under 35 U.S.C. xc2xa7119 of Russian Federation Patent Application Serial No. 2001111055, filed Apr. 16, 2001.
The present invention relates generally to optical fluoride crystals for transmitting below 200 nm UV region light, and particularly to UV region  less than 200 nm transmitting calcium fluoride crystals for excimer laser lithography optics.
The present invention belongs to the technical field of preparing artificial crystals of optical-grade calcium fluoride by growing its monocrystals from a melt, cooling them in a temperature gradient, and using a crystal nucleus for the growth.
Optical-grade fluorite crystals are grown industrially by Stockbarger""s method, in which a container (a crucible) containing the melt is moved through a temperature domain with a fixed gradient under a high vacuum in the presence of directional heat removal, which is generally ensured by a set of shields.
Cylindrical graphite containers with a conical or hemispherical bottom part are generally used for growing optical-grade crystals. Multichamber crucibles, consisting of a set of bowls in the form of a stack, are employed for the large-scale production of crystals, where an orifice is made in the bottom of each bowl to allow the melt to flow through. The number of bowls (vessels) depends partly on the size of the crystal-growing (working) zone of the apparatus and partly on the size of the intermediate crystals that are then to be grown to a larger size.
To grow the crystals in such crucibles, the whole stack of vessels is transferred, inside the crystallization furnace, from the melting zone to a zone where the temperature is kept below the crystallization temperature. The crystals growth begins in the lowest vessel and continues upward in each of the higher ones, as they successively traverse the crystallization isotherm (see pp. 106 and 107 in the Russian book entitled xe2x80x9cOpticheskiy flyuoritxe2x80x9d [Optical-Grade Fluorite] by N. P. Yushkin et al., published by Nauka in Moscow in 1983).
An example of the multicompartment crucible is represented by the apparatus described in a Japanese Patent Application entitled xe2x80x9cPreparation of Calcium Fluoride Crystalsxe2x80x9d (see Japanese Patent Application No. 136515 of the Heisei Era 1.8 (1996), dated May 30, 1996 and published on Dec. 9, 1997/Japanese Patent Applicationxe2x80x94Kokei Seriesxe2x80x94No. 315893 of the Heisei Era 1.9 (1997)). In the apparatus described there, the stack consists of a set of bowls placed on top of one another. Each of these bowls is filled with the charge independently, and the bowls do not communicate with one another. The crystal growth occurs in each bowl separately, and independently of the process taking place in the other bowls.
In the prior-art apparatus that is the most like the object of the present invention is one comprising a xe2x80x9cmultistoryxe2x80x9d crucible, in which disk-shaped fluorite crystals are used as intermediate products for further growth (see p. 107 in the book xe2x80x9cOptical Fluoritexe2x80x9d by Yushkin et al., mentioned above). Each cavity (bowl) in this crucible has at its bottom a conical orifice to allow the melt to flow through from the higher cavity to the lower one, but this orifice is closed in the lowermost bowl.
During the crystallization of the melt, a penetrative growth (intergrowth) takes place through the orifice in the bottom of each bowl, and the small monocrystals thus formed serve as nuclei for the formation of monocrystalline intermediates in the cavities of the upper mold. The chances of a spontaneous formation of a crystal nucleus are practically nil here.
The main problem encountered in the preparation of crystals in the known apparatuses relates to the difficulty of obtaining high-quality optical-grade crystals that have a fixed crystallographic orientation, a low birefringence and a high optical uniformity. A rigorous crystallization regime or system must be used to make this preparation possible. The temperature domain must be so structures as to ensure the absence of any temperature gradients in the charge-melting zone, which guaranteeing a steep temperature gradient in the crystallization zone, and again none in the annealing zone.
In the prior art that is the most like the object of the present invention, the temperature domain with its special structure is created by heaters and a set of reflecting shields used to remove the heat. However, these means do not make it possible to obtain crystals that have the required orientation and a birefringence of less than 5-10 nm/cm.
An aspect of the present invention is to prepare calcium fluoride monocrystals that are oriented in a fixed crystallographic direction and have a low birefringence (less than 5 nm/cm) and a high optical uniformity, by means of a preferred additional removal of heat from the center of crystal formation.
An aspect of the invention is achieved by an improved construction of the multicompartment container, which consists of a) a number of graphite bowls, placed on top of one another, each of which is fitted with a central conical orifice in its bottom part, and b) a seeding until that has a central cylindrical orifice and is mounted under the bottom of the lowermost bowl. Unlike in the most similar prior art, each bowl of the container is fitted here with a heat-removing device in the form of a graphite cylinder. Each of these cylinders has a central conical orifice, is mounted under the bottom of each bowl, and adjoins, with its other surface, a flat piece that has a central conical orifice and serves as the cover or lid for the next bowl down.